Expert: Tom Whiting - 1/15/2010

Question
Mr. Whiting

I have a few questions I would like to ask.

How does a star "die?"  

How do Type I and Type II supernovae occur?

Are the latter a direct result of the "death" of a star?

As always, I greatly appreciate any assistance you may be able to provide.

Answer
Hi Antoniece,
The starting mass of a star determines it's method of death, and not surprisingly, ALL the
other physical properties too... it's size, surface temperature, it's spectral classification,
it's density... all of them.
The most common star, 90% of all stars, are the little red dwarf (Proxima Centauri, Wolf
359, etc) stars.  As they have lifespans of trillions of years, we just don't know how they
die, although we can be confident that they don't explode either... they are too small to begin
with.
Average size stars like the sun can't explode either, they don't have enough starting mass.
Only those very massive, say 5 solar masses and higher, can explode... they constitute only
less than 1% of all stars, but that's still a lot of stars out there, the giants and supergiants.

Stars like the sun go the red giant, planetary nebula, white dwarf, black dwarf route.
Because they don't have the mass to reach what is called "Alpha Capture nuclear reactions"
in their cores... their cores just can't get hot enough. They just don't have enough mass.
They go through the proton-proton chain or CNO Cycle (Hydrogen nuclei fusing to helium nuclei)
then the triple alpha process during the red giant phase (3 He nuclei to a carbon nuclei)
and that's it. The cores do not get hot enough, some 600 million degrees, to pull off the last
nuclear reaction, the alpha capture process.
After these first 2 nuclear reactions, VERY massive stars go through this process in their cores, fusing carbon nuclei with either hydrogen or helium nuclei to form oxygen, nitrogen, silicon, etc. nuclei all the way up to iron nuclei.  Every one of those nuclear reactions releases (gives off, or is exothermic) energy.  But once a massive star has an iron nuclei core, the star is in trouble, because any further nuclear reactions TAKES energy, not gives off energy. (It's endothermic).
So with no further production of energy, there is nothing to support the massive outer layers
from collapse down onto the core. The outer layers come flying down into the super-hot core,
and the star implodes... yes, a supernovae explosion is actually an implosion.
All the elements above iron (gold, silver, uranium, etc) are formed in the final moments as
the star explodes, and that's why those very heavy elements are so rare, compared to the lighter
elements. (Notice the great drop-off on the Periodic Table of all elements heavier than Iron or
Fe).

There are basically two types of supernovae, one is a massive star (Type II) but there are a lot
of binary stars out there too (over half the stars are doubles or multiples). That's where the
Type I supernovae occur. Say a white dwarf is orbiting close to a normal star, and sucking mass
off the bright primary star. There are cases where the little white dwarf sucks in too much
matter for its size, and it explodes, that's a type I supernova.  (The spectrum tells us easily
which type of supernova has occurred).
That, in a nutshell, is the way it works.
The nice thing being, all those supernovae have 'seeded' the original hydrogen/helium mix from
the Big Bang, with all the other heavy elements, #3 Lithium to #92 Uranium...that's where the
heavy elements came from initially, from the cores of supernovae explosions of the past.
In fact, we owe our very existence (at least in the physical sense) to the fact that all those
supernovae occurred before the formation of the Solar System some 4.6 billion years ago.
(And some 9 billion years after the Big Bang)...
If not, there would be no carbon, silicon, oxygen... all the heavy elements. Can't get life
out of just hydrogen and helium gas... which is all the Big Bang gave us.  Supernovae are where
all the elements above helium came from initially.
Clear Skies,
Tom Whiting
Erie, PA  USA